1. Field of the Invention
The present invention relates to an image forming apparatus configured to form a color image of, for example, a copying machine, a printer, or a fax machine. More particularly, the present invention relates to an image forming apparatus capable of forming a detection toner pattern used for reducing color misregistration of toner images of each color.
2. Description of the Related Art
Copying machines and laser beam printers include an image forming apparatus that forms images through various processes by using the electrophotographic method. First, the surface of a photosensitive member is charged by a charging device. Then, a light beam is directed onto the photosensitive member which has been charged. When the light beam is directed onto the photosensitive member, according to the change in the electric potential of the surface of the photosensitive member, an electrostatic latent image is formed on the photosensitive member. The formed electrostatic latent image is developed by using charged toner which is charged to a predetermined charge. The developed toner image is transferred onto a recording medium such as paper, and the toner image transferred onto the recording medium is fixed onto the recording medium by a fixing unit.
Color image forming apparatuses that employ a method called a tandem method are known among the image forming apparatuses that form color images by using toner of multiple colors according to the electrophotography method. The tandem method uses a plurality of photosensitive members in forming the toner images. Each of the photosensitive members corresponds to toner having a different color. Each toner image having a different color formed on each photosensitive member is transferred onto a recording medium when the image is formed. Generally, a toner image formed on each photosensitive member is transferred onto an intermediate transfer member such as an intermediate transfer belt so that a toner image of one color is transferred onto another toner image on the intermediate transfer member. After then, the toner images on the intermediate transfer member are transferred onto the recording medium.
In such a color image forming apparatus, if there is positional misregistration of the toner images of each color transferred from each photosensitive member onto the intermediate transfer member, it may cause color misregistration on the recording medium. This may lead to reduced image quality.
In order to prevent such color misregistration, color misregistration correction control is performed using a toner pattern for color misregistration detection. The color misregistration correction control is a method that forms a toner pattern for color misregistration detection of each color on the intermediate transfer member, calculates a relative amount of misregistration from a difference in detection timing of the toner pattern of each color, and controls the exposure timing and the position of the optical system so that the misregistration is reduced.
The toner pattern for color toner pattern for misregistration detection is detected by an optical sensor arranged in the vicinity of the intermediate transfer member. The color misregistration correction control is performed based on the output from the optical sensor.
The optical sensor includes a sensor that detects diffused reflection light. In detecting a toner pattern for color misregistration detection by using a sensor that detects diffused reflection light, the sensor uses the difference in the level of output. The level of output which is obtained by detecting diffused reflection light reflected from a toner pattern of a chromatic color such as yellow, magenta, or cyan, and the level of output which is obtained by detecting diffused reflection light reflected from the intermediate transfer member are greatly different. Thus, the positional relation of each toner pattern of a chromatic color can be identified by receiving the diffused reflection light.
On the other hand, since toner of an achromatic color such as black toner absorbs light, the amount of light reflected from a toner pattern of an achromatic color is smaller than that reflected from a toner pattern of a chromatic color. Thus, the difference in the level of output of the diffused reflection light reflected from the intermediate transfer member and the level of output of a toner pattern of an achromatic color such as black is too small to be detected unless processing such as increasing the amount of emission is performed.
Thus, Japanese Patent Application Laid-Open No. 2007-156159 discusses a method that uses a combination toner pattern in detecting a position of a toner pattern formed by toner that reflects small amount of light such as black toner. The combination toner pattern includes a chromatic color toner pattern and an achromatic color toner pattern. The image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2007-156159 identifies a position of an achromatic color toner pattern from an output waveform obtained from the combination toner pattern.
FIG. 13A illustrates a model of the toner patterns for color misregistration detection discussed in Japanese Patent Application Laid-Open No. 2007-156159. The detection toner patterns include a yellow toner pattern 1302a, a magenta toner pattern 1303a, a cyan toner pattern 1304a, and a combination toner pattern 1305a, and are formed on an intermediate transfer member 1301. The combination toner pattern 1305a is a black toner image formed over a portion of a magenta toner image.
A light-receiving unit that receives the light reflected from the toner patterns for color misregistration detection outputs an analog signal that includes outputs 1302b, 1303b, 1304b, 1305b1, and 1305b2 that correspond to each of the toner patterns (see FIG. 13B). Each output is converted into a binary digital signal according to a predetermined threshold value indicated by a dotted line in FIG. 13B. The digital signal includes outputs 1302c, 1303c, 1304c, 1305c1, and 1305c2 that correspond to the outputs 1302b, 1303b, 1304b, 1305b1, and 1305b2 (see FIG. 13C).
Further, a central processing unit (CPU) included in the image forming apparatus identifies, for example, the positions of the chromatic color toner patterns from barycentric positions of the outputs 1302c, 1303c, and 1304c included in the digital signal obtained from the chromatic color toner patterns. Further, regarding the combination toner pattern, the CPU acquires a barycentric position of the output 1305c1 and a barycentric position of the output 1305c2 included in the digital signal. Then, the CPU considers that the middle point of the two barycentric positions is the position of the black toner pattern.
Further, as a different method for identifying the position of the black toner pattern, an intersection point of the fall of the output 1305b1 of the analog signal and the threshold value, and an intersection point of the rise of the output 1305b2 and the threshold value are obtained. Then, the middle point of the intersection points can be considered as the position of the black toner pattern.
However, the method using a combination toner pattern such as the one discussed in Japanese Patent Application Laid-Open No. 2007-156159 is not convenient due to the following reasons. Although the positions of the chromatic color toner patterns are identified by using the outputs 1302b, 1303b, and 1304b included in the digital signal, the position of the achromatic color toner pattern (black toner pattern) is obtained by calculating the barycentric positions of the two outputs 1305C1 and 1305C2 included in the digital signal and further calculating the middle point of the two barycentric positions. Thus, in addition to the processing for identifying the position of the chromatic color toner pattern, the apparatus needs to perform processing for identifying the position of the achromatic color toner pattern. Thus, the design of the apparatus becomes complicated.
Further, regarding the other method for identifying the position of the black toner pattern, the intersection point of the rise of the output 1305b1 of the analog signal and the threshold value, and the intersection point of the fall of the output 1305b2 and the threshold value need to be excluded from the detection result. Thus, the design of the apparatus becomes complicated.